In recent years, a greenhouse effect resulting from an increase of a carbon dioxide (CO2) concentration has been pointed out as a cause of global warming phenomena, and there is an urgent need to devise an international countermeasure to protect environment in a global scale. Industrial activities have a large responsibility as a generation source of CO2, and there is a trend to suppress discharge of CO2.
As technologies to suppress the increase of the concentration of acid gas, typically, CO2, there are a development of energy saving products, a separation and recovery technology of discharged acid gas, technologies to use the acid gas as a resource and to isolate and store the acid gas, a switching to alternate energies such as natural energy, atomic energy, and so on which do not discharge the acid gas, and so on.
As separation technologies of the acid gas studied up to now, there are an absorption process, a suction process, a membrane separation process, a cryogenic process, and so on. Among them, the absorption process is suitable for processing a large amount of gas, and its application in a factory, a power station is considered.
Accordingly, a method in which exhaust gas generated when fossil fuel (coal, coal oil, natural gas, and so on) is burned is brought into contact with a chemical absorbent, whereby CO2 in exhaust combustion gas is removed and recovered, and further a method storing the recovered CO2 are performed throughout the world in a facility such as a thermal power station using the fossil fuel. Besides, to remove acid gas such as hydrogen sulfide (H2S) in addition to CO2 by using the chemical absorbent has been proposed.
In general, alkanolamines represented by monoethanolamine (MEA) have been developed from the 1930s as the chemical absorbent used in the absorption process, and they are still used at present. This method is economical and it is easy to increase the removal device in size.
As existing and widely used alkanolamines, there are monoethanolamine, 2-amino-2-methylpropanolamine, methylaminoethanol, ethylaminoethanol, propylaminoethanol, diethanolamine, bis(2-hydroxy-1-methylethyl)amine, methyldiethanolamine, dimethylethanolamine, diethylethanolamine, triethanolamine, dimethylamino-1-methylethanol, and so on.
In particular, ethanolamine being primary amine has been widely used because their reaction rates are fast. However, there are problems that this compound has corrosiveness, is easily deteriorated, and require high energy for regeneration. On the other hand, methyldiethanolamine has low corrosiveness and requires low energy for regeneration, but has a defect that an absorption speed is low. Accordingly, a development of a new absorbent in which these points are improved is required.
In recent years, a study on particularly alkanolamine having structural steric hindrance, among amine based compounds, is vigorously tried as the absorbent of acid gas. Alkanolamine having the steric hindrance has merits that selectivity of acid gas is very high and the energy required for regeneration is small.
The reaction speed of the amine based compound having the steric hindrance depends on a degree of reaction hindrance determined by the steric structure thereof. The reaction speed of the amine based compound having the steric hindrance is lower than that of the secondary amine, for example, such as methylethanolamine and diethanolamine, but higher than that of the tertiary amine such as methyldiethanolamine.
On the other hand, a method in which a cyclic amine being an amine based compound having a structure different from that of alkanolamines is used as the absorbent is also known.